CITRIC ACID 21
Cr03 = 100
Pure chromic acid forms dark ruby-red, silky, acicular crystals, but
that used technically is in much smaller crystals or in red crystalline masses.
The impurities present are mainly sulphuric acid and alkali salts, more
rarely nitric acid and barium and lead salts.
1. Sulphuric Acid.—I gram in 20 c.c. of water should give a clear
solution, which is not rendered turbid by addition of hydrochloric acid
and barium chloride.
2. Alkali Salts.—o-i gram is heated to redness and the residue treated
with water and filtered; if the filtrate is yellow, alkali salts are present.
3. Nitric Acid.—i gram is dissolved in water and the liquid treated
with sulphurous acid solution until it becomes distinctly green and then
with ammonia ; the liquid is boiled and filtered, and the filtrate (which
should be colourless) tested for nitric acid in the usual way.
4. Barium or Lead Salts.—A few grams are treated with water and
a few drops of dilute sulphuric acid; after standing, the clear liquid is
decanted and any residue examined for lead or barium sulphate.
5. Determination of the Chromic Acid.—About o-i gram is boiled
with hydrochloric acid and the liberated chlorine collected in potassium
iodide solution. The iodine set free by the chlorine is then estimated by
means of sodium thiosulphate solution and starch paste (i c.c. of N-thio-
sulphate = 0-03339 gram Cr03).
Even " pure " chromic acid generally contains small proportions of sulphuric
acid or sulphates. In the commercial product as much as 30% of potassium
sulphate lias been found (Kraucli).
C0H807 + HaO = 210
Large, colourless, odourless, non-hygroscopic crystals of acid taste,
soluble in water and alcohol and, to a less extent, in ether. They may
contain tartaric, oxalic and sulphuric acids and salts of calcium, iron and
heavy metals (especially lead and copper).
1. Tartaric Acid.—i gram of the acid is dissolved in 2 c.c. of water
and the solution treated with potassium acetate and alcohol: no turbidity
should be produced. Very small amounts may be detected by dissolving
i gram of the acid in 10 c.c. of distilled water and gradually pouring part
of the solution into 15-20 c.c. of lime water, which should remain clear.
2. Oxalic Acid.—i gram of the acid, dissolved in 10 c.c. of water,
should not be rendered turbid by addition of calcium sulphate solution.
3. Sulphuric Acid and Sulphates.—-The aqueous solution (i: 10)
should not be rendered turbid by addition of barium chloride and hydro-
4. Lime.—The aqueous solution (i: 10), neutralised with ammonia,
should not be rendered turbid by addition of ammonium oxalate.
5. Ash.—10 grams of the acid are carefully burned in a crucible andand empyreumutk*. substances.